In wavelength division multiplexing (WDM) optical communication systems a single optical fiber may be used to carry multiple optical signals. The multiple optical signals are multiplexed to form a multiplexed signal or WDM signal with each of the multiple signals being modulated on separate channels. Each channel may be at an associated wavelength that is separated from adjacent channels by a defined channel-spacing, e.g. according to a channel plan established by the International Telecommunications Union (ITU). The range of wavelengths that may be transmitted on the system is known as the system bandwidth. Systems may utilize their system bandwidth to carry a desired number of channels with desired modulation format and bit rate.
To satisfy increasing demand for transmission capacity in optical transmission systems, spectral efficiency has been increased using a number of techniques. Multi-level modulation techniques and coherent receivers have been used, for example, to allow increased transmission rates and decreased channel spacing, thereby increasing the spectral efficiency (SE) of each channel in a WDM system. In a multi-level modulation format, such as a quadrature amplitude modulation (QAM) format, multiple data bits are encoded on a single transmitted symbol.
While use of multi-level modulation formats may increase spectral efficiency and transmission capacity, such formats may require increased signal-to-noise ratio (SNR). Operating with high SNR requires high optical channel output power and high amplifier pump power, especially for wide system bandwidths. Delivering the required high power levels can present a significant technical and economic challenge, in particular, in undersea systems where the electrical power for the entire cable must be transported along the cable. In this scenario, the ability to realize increased performance may be impeded by a limited amount of available power.
One potential approach to increasing spectral efficiency is to implement spatial division multiplexing (SDM). In an SDM system, a multi-dimensional fiber, e.g. a multi-core or multi-mode fiber, may be used, and the WDM signal may be separated onto each of the dimensions of the fiber. For example, instead of transmitting a WDM signal on a single core fiber, in an SDM system the signal may be separated and transmitted on each of the cores of a multi-core fiber.
Unfortunately, in long-haul SDM systems each of the dimensions, e.g. cores, of the transmission fiber must be amplified. The optical pump power required to pump each of the dimensions of a multi-dimensional fiber may not be available in a power-limited system.
Although the following Detailed Description will proceed with reference being made to illustrative embodiments, many alternatives, modifications and variations thereof will be apparent to those skilled in the art.